The present invention relates to an inflator for generating gas for inflating and deploying an airbag.
As a gas generator for deploying an airbag, a type (combustion type) of inflator which burns a gas-generating agent (propellants) and generates gases by chemical reaction, and another type (stored-gas type) of inflator which ejects a high-pressure gas stored in a container are known.
A stored-gas-type inflator is shown in FIG. 5. FIG. 5 is a schematic longitudinal-sectional view of a known stored-gas-type inflator which is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-250525 (incorporated by reference herein).
The inflator 100 includes a bottle 101 to be charged with a high-pressure gas. A sleeve 109 is connected at an opening 103 of the bottle 101 via a ring 106. The ring 106 is provided with an aperture 106a formed at a central part of the ring 106.
A burst disk or sealing plate 107 is mounted by welding or similar methods to the sleeve 109 side of the ring 106 (i.e., the left side of the ring 106 as shown in FIG. 5). The burst disk 107 is made of steel and has a thickness of approximately 0.3 mm. As shown in FIG. 5, the burst disk 107 bows toward the sleeve 109 side due to the pressure of the gas stored in the bottle 101.
A plurality of gas outlets 104 are formed in the sidewall of the sleeve 109. During operation of the inflator the high-pressure gas in the bottle 101 is ejected through the outlets 104. A housing 110 mates with the sleeve 109 at an end (the left side open end in FIG. 5). The housing 110 includes an initiator fixing or retaining part 110a and a cylinder 110b protruding from the fixing part 110a. The fixing part 110a is affixed and held by the sleeve 109 at the end thereof. An initiator 112 is embedded in the fixing part 110a. An end (right side end) 112a of the initiator 112 extends into the cylinder 110b. 
A piston 115 is disposed in the cylinder 110b of the housing 110. An end 115a of the piston 115 is tapered in a cone-shape. The piston 115 is provided with a hole 115b formed in the rear end of the piston 115. The end 112a of the initiator 112 is inserted into the hole 115b. The burst disk 107 is disposed at a predetermined distance from an end 110c of the cylinder 110b of the housing 110.
The gas outlets 104 of the inflator 100 communicate with an airbag body (not shown). In a normal state, a gas fills the bottle 101 and is sealed in the bottle 101 with the burst disk 107. When the automobile receives an impact, a sensor (not shown) operates and the initiator 112 generates a gas blast. The gas blast moves the piston 115 away from the housing toward the burst disk (i.e., to the right of FIG. 5).
The end 115a of the piston 115 breaks the burst disk 107 at a central part. The high-pressure gas filling the bottle 101 is ejected and is supplied into the airbag body from the inside of the sleeve 109 through the gas outlets 104 formed in the peripheral surface of the sleeve 109.
The end 115a of the piston 115 must be keen-edged so that the piston 115 reliably breaks the burst disk 107. In the above example, the end 115a is formed tapered in a cone-shape.
A gas generator used in an inflator or the like is disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 5-201304, 10-138862, and 12-250525 (all three incorporated by reference herein), in which the piston of the gas generator takes the shape of a cylinder, a cone, and a pyramid (polygonal), respectively.
FIG. 6 shows an end of the piston having another shape. As show in FIGS. 6(A), 6(B) and 6(C), respectively, the end of the piston may be formed as a needle, a cylinder shown, or as a punch.
As shown in FIG. 6(A), the needle 121 includes a fine pointed tip. As shown in FIG. 6(B), the cylinder 123 is provided with a circular recess formed at one end. The circular recess includes a cutting edge 125 around the periphery. As shown in FIG. 6 (C), the punch includes a cylinder 127 having a two-forked end. The forked end includes two cutting edges 129.
From among the shapes of the end of the piston discussed above, the punch-shaped two-forked end is currently thought to be the most effective shape for reliably breaking the sealing plate with the smallest force. The punch-shaped piston cuts into the burst disk at two positions located away from the vertex of the swelling burst disk. However, the two-forked punch-shaped piston has a problem described below.
FIG. 7 is a schematic view of the two-forked punch-shaped piston being deformed at an instant when the piston comes into contact with the burst disk. Although the piston is arranged so that its longitudinal axis 131 is aligned with the vertex of the burst disk swelling in a spherical shape, the tip of each cutting edge 129 is offset to the outside from the axis 131 of the piston. As a result, when the piston comes into contact with the spherically swelling burst disk, the tips of the two cutting edges 129 come into contact with the burst disk at positions remote from the vertex of the disk. The cutting edges do not contact the disk at a right angle and, instead, contact the disk at a smaller angle. As a result, the cutting edges 129 sometimes slide on the surface of the burst disk 129xe2x80x2 and are bent toward the outside, as shown by dotted lines in FIG. 7. Therefore, the cutting edges do not sharply cut and there is a risk that the burst disk is not broken smoothly.
Accordingly, an object of the present invention is to provide an inflator that includes a piston which is capable of reliably breaking a burst disk or sealing plate with a small force.
According to one embodiment of the present invention an inflator is provided. The inflator comprises a bottle to be charged with high-pressure gas and having an opening. The inflator also includes a sealing plate for sealing the bottle at the opening and an initiator for generating a gas blast. The gas blast provides the motive force to break the sealing plate. A punch or piston that includes a cutting edge is provided for breaking the sealing plate. The punch being accelerated toward the sealing plate by the gas blast of the initiator. The sealing plate swells toward the punch by being pressed by the high-pressure gas. The cutting edge of the punch comes into contact with a portion of the sealing plate offset from the vertex of the swell of the sealing plate. The cutting edge of the punch is formed by a tapered face formed at the outer side of the cutting edge (i.e., at the side away from the vertex of the sealing plate).
Due to the tapered cutting edge of the punch, the tip of the cutting edge is positioned inside the periphery of the punch. An angle between the central line of the cutting edge and the surface of the sealing plate is increased. Therefore, the cut made by the cutting edge into the swelling sealing plate is improved, and the deformation of the cutting edge away from the vertex of the sealing plate and the slippage of the cutting edge along the spherical surface of the sealing plate are reduced. As a result, the reliability of the punch breaking the sealing plate is increased.
According to an embodiment of the present invention, the tapered face is preferably formed so as to have an angle smaller than an angle oh of friction with respect to the normal line on a contact point between the cutting edge and the sealing plate, the angle xcex1 of friction being determined in accordance with the materials of the punch and the sealing plate. The length (in the axial direction of the punch) of the tapered face is preferably set to 0.5 mm or greater. With the arrangement of the shape and the size as described above, the cutting edge can effectively and reliably break the sealing plate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.